Project Summary One of the most significant consequences of the opioid epidemic is the increasing prevalence of hepatitis C virus (HCV) among young persons including pregnant women. Pregnancy is a time when women are uniquely motivated to engage in activities which are geared toward improvement of their own health and ensuring the health of their unborn child. As such, pregnant women have frequent prenatal care visits; and health care interventions, such as antiviral therapy and monitoring, can be easily integrated into the existing healthcare infrastructure of prenatal care. The benefits of HCV treatment are numerous, including prevention of severe liver disease, hepatocellular carcinoma, and liver transplantation, as well as improvements physical and emotional health. The antenatal period represents an ideal window of opportunity for treatment of HCV in pregnancy due to increased health care utilization and prevention of perinatal HCV transmission of HCV. Our team conducted the first study of HCV treatment during pregnancy with the directly-acting antiviral medication ledipasvir/sofosbuvir (LDV/SOF). Nine women enrolled and began a 12-week course of LDV/SOF. All the participants were cured of HCV and all infants are HCV negative to date without any significant pharmacokinetic (PK) changes with LDV or SOF administered during pregnancy. Though infant follow-up is still ongoing, no maternal or neonatal safety concerns have been identified. Ideally, these positive results would lead to a larger study of LDV/SOF treatment during pregnancy. However, LDV/SOF is limited because it is only approved for use with HCV genotypes 1, 4, 5, and 6, due to lack of efficacy in genotypes 2 and 3. Thus, to conduct a large-scale effectiveness trial of HCV treatment during pregnancy, a PK and safety study of a pan-genotypic regimen is needed. Given the reassuring safety data for sofosbuvir in the previous study with LDV/SOF, sofosbuvir 400mg/velpatasvir 100mg (SOF/VEL) is the ideal candidate for the pan-genotypic regimen for treatment of hepatitis C during pregnancy. Safe administration of drugs in pregnancy may require dose adjustment due to the pregnancy-induced physiologic alternations. Therefore, careful pharmacokinetic (PK) evaluation is a critical first step to ensure safe administration of drugs to both the mother and the developing fetus. In this R21 application, we propose a single-arm, single-center, open label Phase 1 evaluation of the PK and safety of treating HCV with a 12-week course of SOF/VEL in 10 HCV-infected pregnant women. Therapy will be initiated at approximately 24 weeks of gestation. In this study we will determine: 1) if the PK of the SOF and VEL are similar in pregnancy as compared to those in nonpregnant women, 2) the viral response to SOF/VEL treatment in pregnancy, and 3) if there are any initial maternal or neonatal safety concerns detected with antenatal SOF/VEL administration compared with HCV-infected controls delivered at our institution. From the findings of this first study, larger studies can fully evaluate efficacy and safety and thus optimize treatment of HCV during pregnancy.